A British-built spacecraft is set to blast off from the Earth bound for Mercury, the mysterious planet closest to the sun.

BepiColombo, one of the most ambitious missions ever undertaken by the European Space Agency (Esa), will send two orbiters to explore the hellish world where surface temperatures reach 450C.

One probe was built by satellite makers Airbus Space and Defence at its assembly centre in Stevenage, Herts. The other was constructed in Japan.

Infographic showing the different components of BepiColombo (Airbus Defence and Space/PA)

The mission, launching on October 20, has cost an estimated 1.6 billion Euros (£1.4 billion).

Unlike any other interplanetary spacecraft in history, BepiColombo carries a futuristic ion electric propulsion drive, also designed and built in the UK.

Test firing one of BepiColombo’s T6 ion thrusters (Nasa/JPL/PA)

Four ion engines on the Mercury Transfer Module (MTM) transporting the orbiters will provide thrust by electrically ejecting a “plasma” of charged xenon gas.

Although the force an ion motor produces is very small – far less than that of a chemical rocket – it can be kept firing for a long period of time.

Mission controllers preparing for the BepiColombo launch at the European Space Operations Centre (Esoc) in Darmstadt, Germany (Esa/PA)

The four-tonne spacecraft will be launched into an “escape trajectory” orbit by Esa’s most powerful rocket, the Ariane 5, from the European spaceport at Kourou, French Guiana.

It will then set off on a seven-year 8.5 billion kilometre (5.2 billion mile) journey involving a complex series of gravity-assist fly-bys around the Earth, Venus, and Mercury.

Image of Mercury taken by Nasa’s Messenger spacecraft (Nasa/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/PA)

As it circles towards the sun, BepiColombo will use its ion drive and the fly-bys not to build up speed but to brake and overcome the star’s powerful gravity.

After playing tag with Mercury six times while crossing the planet’s orbital path, the spacecraft will arrive at its destination in 2025.

Esa’s Mercury Planet Orbiter (MPO) and the Japanese space agency Jaxa’s Mercury Magnetospheric Orbiter (MMO) will then separate to study the little-known sun-baked world for up to two years.

MPO will make global maps of Mercury’s surface chemistry and geological features, while MMO investigates the planet’s internal structure and magnetic field.

One of MPO’s key instruments, the Mercury Imaging X-ray Spectrometer (Mixs), was designed and built at the University of Leicester.

Only two spacecraft have previously visited Mercury. Nasa’s Mariner 10 flew past the planet three times in 1974-75, and the American space agency’s Messenger probe orbited Mercury from 2011 to 2015, taking photos of the surface.

Dr Jerry Bolter, project manager at Airbus Defence and Space, said: “The only other spacecraft to go in orbit around Mercury was Nasa’s Messenger. That was a very, very light spacecraft and no-where near as capable as Bepi will be. The scientists describe Messenger as the hors d’oeuvre and Bepi as the main course.”

Describing the spacecraft’s ion drive, supplied by British defence tech company QinetiQ and installed at Airbus, he said: “If we relied on chemical propulsion then we’d need 17 tonnes of propellant.

“The ion drive needs just 581 kilograms of propellant and does the equivalent of 17.8 million miles to the gallon.”

Two ion engines run at any one time, producing a maximum thrust of 290 millinewtons – the equivalent of about an ounce of force. They will be actively firing for four-and-a-half years, more than half the total journey time.

Dr Bolter added: “I’ve been working on this project since 2006. It will be very emotional I think. It’s a fundamental milestone getting the spacecraft off the ground. Then you start worrying: will we get communication? Will we get power? But it’s also exciting.”

A key challenge for mission scientists was coping with searing temperatures of more than 350C.

In Greek mythology, Icarus flew too close to the sun and paid the ultimate price. Likewise, BepiColumbo’s orbiters would face destruction without adequate protection.

Among the measures employed were a heat shield, multi-layers of novel ceramic and titanium insulation, and ammonia-filled “heat pipes” that conduct heat to a radiator face always pointed away from the sun.

Only the MMO will spin to reduce overheating on any one surface.

“We have an environment inside the spacecraft where the electronics can run at normal temperature,” said Dr Bolter.

Mixs scientist Professor Emma Bunce said: “There are some interesting quirks about Mercury that we still don’t understand. Messenger told us a great deal but also raised more questions.

“It’s extremely exciting but also a little bit terrifying.”

BepiColombo is named after the late Guiseppe “Bepi” Colombo, an Italian scientist and engineer from the University of Padua who played a leading role in the 1974 Mariner 10 mission to Mercury.